The present invention relates to a surface elastic wave filter having a plurality of input and output interdigital transducers which are alternately arranged, e.g., arranged such that the output interdigital transducers are disposed between the input interdigital transducers. More specifically, the present invention relates to a surface elastic wave filter having a plurality of input and output interdigital transducers, the input and/or output interdigital transducers comprising either finger-withdrawn interdigital transducers which have electrode fingers withdrawn in different positions and different frequency blocking ranges, so that the surface elastic wave filter has desired better frequency blocking ranges and a low insertion loss.
Known surface elastic wave filters with desired amplitude and phase characteristics designed substantially independently of each other include apodized interdigital transducers used as weighted transducers for one of input and output interdigital transducers. Such surface elastic wave filters are subject to ripples created in their passband due to the electric reexcitation effect. In order to reduce the ripples, the surface elastic wave filters are used with a relatively large insertion loss of 20 dB, for example.
One effective approach to improve the insertion loss, i.e., to lower the ripples and the insertion loss, is to employ a number of input and output interdigital transducers with the output interdigital transducers disposed between the input interdigital transducers. Since the output interdigital transducers are positioned between the input interdigital transducers, a surface wave produced from the input interdigital transducers by bidirectional excitation is applied to the output interdigital transducers. Reradiated waves owing to electric reexcitation and passed waves cancel each other, so that any reflective waves responsible for ripples in the passband will not be generated. The surface elastic wave filter which is designed through the above technical approach for a lower insertion loss is, however, unable to reach a desired large level of attenuation in the frequency blocking ranges because of increased electric reexcitation. To avoid such a drawback, the interdigital transducers employed in the surface elastic wave filters are weighted.
Since apodized interdigital transducers suffer a large weighting loss and electric reexcitation, finger-withdrawn interdigital transducers, i.e., interdigital transducers from which electrode fingers ar selectively withdrawn, are used as input and output interdigital transducers.
According to one conventional scheme, a surface elastic filter comprises a plurality of uniform input interdigital transducers having 58 interdigitating electrode fingers (i.e., 29 electrode finger pairs) and a plurality of finger-withdrawn interdigital transducers having 25 electrode pairs with selected electrode fingers withdrawn in the same positions. Actually, four such output interdigital transducers are disposed between five such input interdigital transducers. The five input interdigital transducer have respective signal input connectors and respective ground connectors which are connected parallel to each other and also to input terminals. Likewise, the four output interdigital transducers have respective signal output connectors and ground connectors which are connected parallel to each other and also to output terminals. In operation, a high-frequency signal is supplied to the input terminals, and a signal derived from the high-frequency signal and having frequency characteristics depending on the characteristics of the input and output interdigital transducers is extracted from the output terminals.
The above conventional surface elastic filter is weighted such that the five uniform input interdigital transducers and the four finger-withdrawn output interdigital transducers are alternately arranged. The prior weighted surface elastic filter is however disadvantageous in that high-level sidelobes are produced in the frequency blocking ranges and hence desired effective frequency blocking ranges cannot be achieved.